This application claims the benefit of Korean Patent Application No. 2000-81491, filed on Dec. 26, 2000 in Korea, which is hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a method of manufacturing a cholesteric liquid crystal (CLC) color filter. The CLC color filter is often used in flat panel displays such as liquid crystal display devices.
2. Discussion of the Related Art
Thin film transistor liquid crystal display (TFT-LCD) devices are commonly used for liquid crystal display devices because of its superior reproduction of color images and thin size. The conventional thin film transistor liquid crystal display device includes an upper substrate (a color filter substrate) and a lower substrate (an array substrate) facing each other. A back light unit is located under the lower substrate in the conventional thin film transistor liquid crystal display devices. Because only about 7% of light irradiated from the back light unit reaches the display screen through liquid crystal cells, the back light unit needs to be brighter to obtain higher brightness for the liquid crystal display, which leads to a higher power consumption. Accordingly, batteries having large capacitance and heavy weight have been used to supply enough power to the back light unit. However, these batteries are still limited in terms of their duration time. To overcome the above-described problems, reflective type liquid crystal display devices have been considered. Because the reflective type liquid crystal display devices use ambient light for a light source, the power consumption of the back light unit can be decreased dramatically. Accordingly, the reflective type liquid crystal display devices are usually used for portable electronic devices such as a personal digital assistant (PDA) that can be driven for long hours.
A pixel region of the reflective type liquid crystal display device is made of an opaque reflector or an opaque reflective electrode, whereas the pixel region of a transmissive type liquid crystal display device is made of a transparent electrode. However, because the reflective type liquid crystal display device uses the ambient light for the light source, the brightness of the display is very low. The ambient light passes through the color filter substrate and is reflected by the reflective electrode on the lower substrate. Thereafter, the light passes through the color filter substrate again to display color images. The ambient light loses most of its brightness during its double passage through the color filter substrate. The transmittance characteristics of the color filter should be improved to overcome the low brightness problem in the reflective type liquid crystal display device. To this end, color purity needs to be lowered to increase the transmittance of the color filter. However, there is a limit to increasing the brightness by lowering the color purity.
Liquid crystal display devices using cholesteric liquid crystal (CLC), which selectively reflects or transmits an incident light, have been developed to improve the liquid crystal display devices. Generally, liquid crystal molecules have liquid crystal phase depending on the structure and composition of the liquid crystal molecules. The liquid crystal phase is affected by temperature and concentration. Nematic liquid crystal which has liquid crystal molecules regularly aligned in a certain direction, has been researched and applied widely in the liquid crystal display field. The nematic liquid crystal is commonly applied to liquid crystal display devices. The cholesteric liquid crystal (CLC) has twisted molecular axes or twisted directors of nematic liquid crystal from mixing the nematic liquid crystal with molecules having chiral characteristic, which means that a molecular structure of the liquid crystal does not superimpose on its mirror image. Generally, the nematic liquid crystal phase has regularity in that the liquid crystal molecules are aligned in a certain direction. On the other hand, the cholesteric liquid crystal (CLC) has a layered structure and the liquid crystal molecules in every layer show similar regularity to that of the nematic liquid crystal. However, the alignment of the liquid crystal molecules of each layer rotates in a certain direction, which can be clockwise or counterclockwise, and thus causing a difference in the reflectance between layers. A color can be displayed by a reflection and an interference of light that are caused by the difference in the reflectance between the layers. The rotations of the cholesteric liquid crystal (CLC) molecules form a helical structure. The two most important characteristics in the helical structure of the cholesteric liquid crystal (CLC) are rotational direction and pitch, i.e., period for 360 degrees rotation of the liquid crystal molecules. That is, the pitch can be understood as a distance between the first cholesteric liquid crystal (CLC) layer and the last cholesteric liquid crystal (CLC) layer when the cholesteric liquid crystal (CLC) molecules in the first cholesteric liquid crystal (CLC) layer rotate 360 degrees. The pitch is a parameter that decides the hue of the cholesteric liquid crystal (CLC). That is, if the pitch is the same as the wavelength of red color, i.e., 650 mn, the cholesteric liquid crystal (CLC) reflects the red color observed in the front direction. If the light reflected from the cholesteric liquid crystal (CLC) is observed in an angle to the plane of the color filter substrate, all colors such as yellow, green and blue, for example, which are included in a region of visible light, can be seen depending on the viewing angle. If the cholesteric liquid crystal (CLC) is used for flat display devices, which use transmission and scattering phenomenon to display images, a color image can be displayed using reflection and scattering phenomenon of a particular color. Another important characteristic in the helical structure of the cholesteric liquid crystal (CLC) is the rotational direction of the CLC helix. The rotational direction of the CLC helix is an important factor for the polarization phenomenon. That is, the direction of a circular polarization of the reflected light depends on whether the helix structure of the cholesteric liquid crystal (CLC) is right-handed or left-handed. The right-handed cholesteric liquid crystal (CLC) reflects a right circular polarization that has a wavelength corresponding to the pitch of the right-handed cholesteric liquid crystal (CLC). Because the ambient light is a mixture of a right circular polarization and a left circular polarization, the right circular polarization or the left circular polarization can be extracted according to the structure of the cholesteric liquid crystal (CLC), i.e., a right handed helix or left-handed helix. Because polarization property, i.e., a linear polarization, is used in the conventional liquid crystal display devices, the degree of light utilization will be greatly improved using the cholesteric liquid crystal (CLC), and will result in an effective reduction of power consumption compared to the color filters including pigment or dye.
The conventional manufacturing method of a cholesteric liquid crystal (CLC) color filter will be described hereinafter with reference to drawings attached herein. FIGS. 1A to 1D are cross-sectional views illustrating a conventional manufacturing sequence of a cholesteric liquid crystal (CLC) color filter.
In FIG. 1A, an alignment layer 10 is coated on a transparent substrate 1. A polyimide-based resin is usually used as the material for the alignment layer because it has excellent alignment characteristics with various liquid crystal materials and is suitable for the liquid crystal material. The coated alignment layer 10 then undergoes a heat-curing process.
In FIG. 1B, the surface of the alignment layer is rubbed by a rubbing fabric. The surface of the cured alignment layer is rubbed by the rubbing fabric in order to make scratches or grooves in a uniform direction. This rubbing process is needed in order to provide uniform alignment of the liquid crystal molecules and thus provide a display with uniform characteristics.
In FIG. 1C, the cholesteric liquid crystal (CLC) layer 14 is formed on the rubbed alignment layer 10. In FIG. 1D, the cholesteric liquid crystal (CLC) color filter 16 is formed through a light exposing process in which the pitch of the cholesteric liquid crystal (CLC) is controlled by light exposure, i.e., coloring. The cholesteric liquid crystal (CLC) can be classified into thermochromic CLC, monochromic CLC and photochromic CLC. The coloring method depends on these CLC types. Because a color of the thermochromic CLC depends on temperature, the cholesteric liquid crystal (CLC) color filter can be formed through an ultraviolet ray curing process when the color of the thermochromic CLC reaches a desired color by varying the temperature. The color of the cholesteric liquid crystal (CLC) means a color seen when the light is transmitted through the cholesteric liquid crystal (CLC) by controlling the pitch to correspond to a wavelength range of the desired color. Because the thermochromic CLC makes it possible to cure only desired portions of the thermochromic CLC using ultraviolet ray, it is easy to form a pattern. However, it is not easy to equip a light exposure apparatus for heating. The monochromic CLC shows only one color and thus it is not hard to equip a necessary apparatus because the temperature variation is unnecessary. However, because the patterning process using a photolithographic technique is needed in the case of the monochromic CLC, an extra process is required for the monochromic CLC. The photochromic CLC changes its color depending on light exposure degree of ultraviolet ray having a particular wavelength. Accordingly, an additional photolithographic process for patterning is not necessary here because the pattering process can be carried out by just controlling the degree of light exposure to desired portions of the photochromic CLC.
In the conventional manufacturing method of cholesteric liquid crystal (CLC) color filters, the rubbing process is carried out independently of the coloring process and accordingly, production yield is decreased because of an added process. In addition, electrical and mechanical defects caused by friction may occur during the rubbing process, which results in problems such as random phase distortion or light scattering.
Accordingly, the present invention is directed to a manufacturing method of cholesteric liquid crystal (CLC) color filters that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a manufacturing method of cholesteric liquid crystal (CLC) color filter to increase a production yield and to lower a production cost.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of manufacturing cholesteric liquid crystal (CLC) color filters comprises forming an alignment layer on a transparent substrate, forming a cholesteric liquid crystal (CLC) layer on the alignment layer, controlling a pitch of a cholesteric liquid crystal (CLC) helix and forming an alignment treatment on the alignment layer simultaneously by irradiating the substrate with ultraviolet ray. The alignment layer is made of a photosensitive alignment material. Cholesteric liquid crystal (CLC) molecules have a helical structure and are photosensitive. The cholesteric liquid crystal (CLC) is a photochromic CLC.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.